JOE TUCCIARONE / SCIENCE PHOTO LIBRARY JOE TUCCIARONE / SCIENCE PHOTO LIBRARY
Brown dwarf. Artwork of a planet-like brown dwarf "star" on the edge of a globular cluster of stars, seen from a nearby moon. Brown dwarves may be part of dark matter, the invisible material that makes up about 90% of the universe's mass. Brown dwarves are classed as MACHOs (Massive Compact Halo Objects) because they are likely to have formed on the periphery (halo) of a galaxy where material is scarce. A brown dwarf is an object which began to accumulate material like a star but which never collected enough for nuclear reactions to begin in its core. Brown dwarves are difficult to detect because, like a planet, they can only be seen by the light they reflect from nearby stars.
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